Circuit breakers are commonly used for protecting electrical circuits and devices from damage due to circuit overloads and ground faults. Circuit breakers embodying thermal and magnetic mechanisms have heretofore been used to trip the circuit breaker upon over current conditions. Some of these earlier devices included interchangeable trip units so as to provide trip current ratings that could be changed without changing the entire circuit breaker.
Furthermore, shunt trip mechanisms have been developed to co-ordinate and provide interaction between circuit breakers on complex electrical distribution circuits so that circuit breakers on different parts of the electrical system would trip at different levels and different time delays upon the occurrence of electrical faults.
For example, U.S. Pat. No. 3,826,951 disclosed a shunt trip mechanism which utilized electronic circuits to analyze an electrical signal produced by a sensing device such as a current transformer to generate a trip command to a shunt trip mechanism upon occurrence of an electrical fault.
Circuit parameters which have heretofore been monitored in order to activate such trip devices include:
(a) the level of fault current
(b) ground fault detection
(c) the maximum level of continuous current the circuit interrupter can withstand without damage
In order to minimize the cost in providing different circuit interrupters, various types of circuit interrupters having mechanical interchangeable circuit configurations have been developed.
For example, U.S. Pat. No. 4,210,887 discloses a circuit interrupter having interchangeable trip circuits removably disposed in a circuit interrupter housing and connected between a current transformer and a trip mechanism and includes rejection structure to ensure that only trip unit housing circuitry compatible with the electrical characteristics of the circuit interrupter can be inserted into the housing.
Some devices utilize several current transformers in order to monitor a range of electrical characteristics whereby each current transfer used in association with the electrical device provide a portion of the range of electrical characteristics monitored. Other devices utilize several devices in association with current transformers in order to monitor a range of electrical characteristics of an electrical system whereby each device provides a portion of the range of electrical characteristics monitored.
Such structures, however, present relatively complicated devices.
It is an object of this invention to provide a programmable logic control which is simple to construct and use.
It is a further object of this invention to provide a programmable logic control which has greater accuracy and selectivity than other circuit interrupting devices.